I. Field of the Invention.
This invention relates to compositions and to methods for controlling the gelation time of aqueous silicate-containing compositions and further relates to methods wherein said aqueous silicate-containing compositions are utilized in treating a subterranean formation.
II. Description of the Prior Art.
In the production or treatment for the purposes of improving production of fluids from a permeable subterranean formation, it is often helpful and sometimes necessary to lessen the permeability of portions or zones of the formation. This entails sealing or plugging certain interstitial spaces, channels, fissures, and the like in order to seal off, plug, or otherwise obstruct openings providing fluid passageways in the permeable formation penetrated by a well bore.
Aqueous silicate solutions which are gelled or solidified with acid producing activators have been known and used in many applications including sealing and consolidation processes carried out in subterranean formations. A number of water-soluble silicates and processes using such silicates are disclosed in U.S. Pat. Nos. 3,202,214 and 3,435,899 which are assigned to the assignee of the present invention and are hereby incorporated by reference.
Sodium silicate is a complicated system of various molecular weight silica polymers in an alkaline solution. Aside from requiring a certain minimum amount of alkalinity, sodium silicate has no definite chemical combining numbers. When sodium silicate is acidified to a pH less than about 10 or 11, the sodium silicate is converted partially to silicic acid. Silicic acid exists at these alkaline pH's as a weak acid. Instead of precipitating and producing silica, SiO.sub.2, the silicic acid remains hydrated and forms a three-dimensional network entrapping the solvent water. This network is a gel since both phases are continuous. A slight lowering of the pH brings about radical changes in gel time. Consequently, gel times are difficult to control, and lumping from local acid concentrations during large scale mixing frequently occurs.
Sodium silicate may be caused to form a gel which will seal an underground formation or plug capillaries therein by the addition of polyvalent cations such as Ca.sup.++, Fe.sup.+++, Cr.sup.+++, Cu.sup.++, etc. However, the addition of the polyvalent cations described above utilizing salts containing the cations may cause immediate gelation of the sodium silicate which allows no time to pump the solution into the formation.
U.S. Pat. No. 3,202,214 discloses a method of gelling an aqueous silicate composition utilizing a reducing agent and an oxidizing agent. Although method dissolved therein is useful in many application, the use of oxidizing and reducing agents to gel an aqueous sodium silicate-containing composition results in increased expenses and the gel produced therefrom undergoes syneresis and is thus not totally satisfactory for use in treating a subterranean formation. Furthermore, the gels formed by the use of an oxidizing agent and reducing agent are not particularly effective in treating subterranean formations having temperatures between 120.degree. F. and 170.degree. F.
The present invention provides a method of gelling aqueous silicate compositions through the use of thermally responsive gelation activators which overcomes or at least mitigates the above described problems.